1. Field of the Invention
This invention relates, in general, to an AC-DC converting apparatus which is employed with an inverter apparatus, and is used as a DC power supply device. In particular, the invention relates to an AC-DC converting apparatus wherein an output current is controlled in accordance with an input voltage waveform to improve the power factor of the AC power source.
2. Description of the Prior Art
One example of an AC-DC converting apparatus used as a DC power supply is disclosed in a thesis entitled "Power Factor Improvement of Single Phase Rectifying Circuit by a Chopper" presented at an joint convention in 1983 of the Hokuriku branch of four Electric learned societies. This thesis discloses an AC-DC converting circuit wherein a sine waveform proportional to an input AC voltage is obtained through a transformer, and an output current is controlled in accordance with a pulse waveform obtained by rectifying the sine waveform to achieve a high power factor, as shown in FIG. 1. In FIG. 1, a sine wave of an AC power supply 21 is applied to terminals 23a and 23b of a four-diode bridge 23, and the output of bridge 23 is applied to a DC reactor 25 through a harmonic filter 27 composed of a coil 27a and a capacitor 27b. The current Id from bridge 23 is controlled by an NPN transistor 29. When transistor 29 is energized, current Id flows through reactor 25 and transistor 29, thereby storing energy into reactor 25. When transistor 29 is de-energized, reactor 25 releases the energy, and generates a relatively high voltage Vo which is applied to a load L through a diode 31. If current Id is controlled in accordance with a reference value having a fully-rectified waveform of the input voltage Vi of power supply 21, the input current I becomes a sine wave, and the power factor thereof approaches 1.
To accomplish the above-described operation, the above-described main circuit is provided with a waveform detection circuit 35, a constant voltage control circuit 37 and a swiching control circuit 39. Waveform detection circuit 35 includes a transformer 41 for detecting the waveform of input voltage Vi. A primary winding of transformer 41 is connected to power supply 21. A secondary winding of transformer 41 is connected to a full-wave rectifier 43, and a center point thereof is grounded. A full-wave rectified signal Vd obtained by a grounded resistor 45 is fed to a multiplier 47 of constant voltage control circuit 37. A voltage deviation signal Ve is generated by a subtractor 49 in accordance with an output voltage Vo obtained between opposite ends of a resistor 51 connected in parallel with capacitor 33, and a prescribed output voltage reference Vr. The above-described rectified signal Vd is multiplied by deviation signal Ve fed through a gain regulator 53 in multiplier 47, thereby generating an output current reference Ir. The current Ir has a pulse waveform directly proportional to the rectified waveform of input voltage Vi. A subtractor 55 of switching control circuit 39 generates a deviation signal between the output current reference Ir and an output current Id detected by a current detector 57. The detector 57 is connected between capacitor 27b of harmonic filter 27 and the emitter of transistor 29. Transistor 29 is switched on and off by a comparator 59 having hysteresis in accordance with the deviation signal. Thus, the output voltage Vo is controlled at a prescribed voltage value, and the phase of the waveform of the output current Id is matched to the phase of the pulse waveform, directly proportional to the rectified waveform of input voltage Vi. As a result, the power factor of the apparatus may be maintained at approximately 1.
However, in the AC-DC converting apparatus shown in FIG. 1, since transformer 41 is used for detecting the waveform of the input voltage Vi, a space for arranging the transformer is needed in the apparatus. Furthermore, since the transformer may also be used for supplying a voltage to another circuit, the detected waveform of the input voltage Vi often may be distorted.
Another example of an AC-DC converting apparatus is disclosed in U.S. Pat. No. 4,437,146 issued to Ralph F. Carpenter. In this device, the waveform of the input voltage may be detected without using a transformer. However, since a plurality of resistors are used for detecting the waveform of the input voltage, it is not easy to determine a constant value of the detecting circuit in order to avoid distortion of the detected waveform. Furthermore, the power factor correction circuit, including the detecting circuit, is not isolated from the power supply in the Carpenter apparatus.